Coherent optical communications enable data to be transmitted over long haul (typically >2,000 kin) optical transmission networks at higher data rates. A coherent detector requires that the received phase modulated optical signal is digitized for a digital signal processor (DSP). Digitizing the received signal requires a high speed analog-to-digital converter (ADC). However, an increase in serial ADC sampling rates is difficult to implement for single-wavelength coherent detection at data rates greater than 100 Gbit/s.
A super-channel is an evolution in dense wavelength division multiplexing (DWDM) in which multiple coherent optical carriers are combined on a single super-channel at a higher data rate, and is brought into service in a single operational cycle. Rather than a single wavelength channel of, e.g., 100 Gbit/s, a super-channel a multi-wavelength signal in which each wavelength operates as a sub-channel at the maximum data rate permitted by ADC components.
One notable difference between the super-channel and conventional wavelength division multiplexing (WDM) is a size of a gap between frequencies of different sub-channels. The super-channel can decrease the size of the gap between wavelengths of the sub-channels, but the transmitted data is handled uniformly, i.e., each super-channel is handled like a single wide channel, in terms of adding, dropping, and routing data. Various techniques can be used to reduce the sub-channels gap to the MHz range. These techniques include orthogonal-band-multiplexed (OBM), orthogonal frequency division multiplexing (OFDM), no guard interval (NGI)-OFDM, Nyquist WDM, and multi-channel equalization (MCE)-WDM.
However, the small size of the gap between frequencies of different sub-channels cause nonlinearity of noise in the sub-channels, i.e., each sub-channel receives different nonlinearity-induced phase noise causing non uniformity in the bit-error-ratio (BER) across the sub-channels. One solution to this problem is to increase the transmission power over the sub-channels experiencing higher noise. However, the increase of the transmission power for one channel also changes the nonlinearity-induced distortion of other channels. Therefore, it is difficult to adjust the power levels of all the sub-channels in the same fiber to meet the BER requirements.
Accordingly, there is a need for a system and a method for transmitting data over an optical super-channel formed by a set of sub-channels in a single fiber such that the transmission over all the sub-channels meets the BER requirements.